1. Field of the Invention
The present invention relates to a packet transfer apparatus and particularly relates to a technique for achieving power saving in a packet transfer apparatus which transfers multicast packets.
2. Description of the Related Art
The number of IP routers (hereinafter referred to as routers) provided in carrier networks and corporate networks has increased rapidly with the popularization of the Internet, the accelerated adoption of IP technology in landline telephones and the accelerated adoption of IP technology in corporate networks. The performances of the routers for transferring traffic have been improved as the amount of network traffic has gone on increasing. Routers' power consumption has increased year by year with the popularization of the routers and improvement of the routers' performances. Techniques for reducing routers' power consumption have been developed in response to recent ecological issues, etc.
A method of reducing the clock frequency or power supply voltage in a packet transfer portion of a router to reduce transfer performance is one of techniques for reducing power consumption. For example, a power control function and a power-off function have been described in “AX6700S/AX6300S Software Manual Configuration Settings, Vol.1”, ALAXALA Networks Corporation, November 2006, pp. 181-182. According to these functions, packet relaying performance can be reduced or power supply to unused modules can be stopped to reduce router's power consumption when the amount of traffic is small.
A method of changing the speed of a network interface according to the amount of traffic is another technique for reducing power consumption. An example of this technique has been described in “Rapid PHY Selection (RPS): A Performance Evaluation of Control Policies”, Ken Christensen, January 2007. The speed of a PHY (physical chip) in an Ethernet (registered trademark) interface can be changed according to the amount of traffic to thereby reduce power consumption.
On the other hand, use of IP multicast has advanced to achieve contents distribution such as video distribution to a plurality of receivers on an IP network. In IP multicast, contents are transmitted to a group. A client which wishes to receive contents transmits a message for joining the group to an upstream router near a server which distributes the contents. Upon reception of the joining message from the client, the router transmits the group joining message to an upstream router nearer to the distribution server to start reception of multicast packets from the upstream router. The router transmits the received multicast packets to the client, so that the client can start reception of the multicast packets (contents).
On the other hand, when the client has no need to receive contents, the client can terminate contents reception by transmitting a group disjoining message to the upstream router. Upon reception of the disjoining message, the router can stop multicast packet transfer to the client. When there is no client wishing to receive contents under the router, the router can stop multicast packet reception by transmitting a multicast disjoining message to the upstream router.
The present router stands by while always holding the maximum transfer capability to perform multicast packet transfer regardless of whether a multicast joining message is received or not. This is because the router does not know when a client transmits a multicast joining message.
The specification of joining and disjoining messages exchanged as router-client messages in an IPv4 network has been defined as a protocol called IGMPv3 in IETF RFC3376, Internet Group Management Protocol, Version 3. The specification of the messages defined in this protocol has been described chiefly in “5. Description of the Protocol for Group Members” and “6. Description of the Protocol for Multicast Routers”.
The specification of disjoining/joining messages in an IPv6 network has been defined as a protocol called MLDv2 in IETF RF3810, Multicast Listener Discovery Version 2 (MLDv2) for IPv6. The specification of the messages defined in this protocol has been described chiefly in “6. Protocol Description for Multicast Address Listeners” and “7. Protocol Description for Multicast Routers”.
The specification of joining/disjoining messages exchanged as router-router messages has been defined as a protocol called PIM-SM in IETF RFC4601, Protocol Independent Multicast—Sparse Mode (PIM-SM): Protocol Specification (Revised).
As described in the background art, a router in a multicast network receives a group joining message from a client or a downstream router and transmits the group joining message to an upstream router before the router starts multicast packet transfer. On the other hand, the router stands by while holding 100% transfer capability even in a state in which there is no group joining message received from a client, that is, in a state in which multicast packet transfer is stopped. That is, the router in a standby state consumes electric power not lower than electric power necessary for traffic transfer.
If the transfer capability or interface speed of the router is reduced in accordance with the amount of traffic input/output to/from the router when traffic transfer is stopped or when the amount of transfer is small, power saving can be achieved. It is however difficult to predict the amount of traffic flowing in a network because the amount of traffic generally changes temporally and has bursting characteristic. It is therefore necessary to perform transfer performance control in accordance with the amount of traffic in order to achieve power saving in the router.
In the method of controlling the packet transfer performance of the packet transfer portion as described in “AX6700/AX6300S Software Manual Configuration Settings, Vol. 1”, ALAXALA Networks Corporation, November 2006, pp. 181-182, the administrator of the apparatus enters commands into the router based on statistic information to control transfer performance. For this reason, it is difficult to control transfer performance meticulously following the amount of traffic.
In the method of changing the speed of PHY (physical chip) of the Ethernet (registered trademark) interface as described in “Rapid PHY Selection (RPS): A Performance Elevation of Control Policies”, Ken Christensen, January 2007, the duty factor of the buffer is described as an example of a turning point for changing the speed. In this case, when burst traffic occurs, packet loss is produced at the time of changing from a low-speed PHY to a high-speed PHY if the quantity of the buffer is too small.
Incidentally, in either “AX6700/AX6300S Software Manual Configuration Settings, Vol.1”, ALAXALA Networks Corporation, November 2006, pp.181-182, or “Rapid PHY Selection (RPS): A Performance Elevation of Control Policies”, Ken Christensen, January 2007, there is no description about electric power control interlocked with multicast joining messages.